Patent GB-707517-A (GB)
Title: Improvements in or relating to electric pulse communication systems
Inventor: TREADWELL CYRIL GORDON
IPC: H04J3/00
Filed: 1952-09-05
Abstract: 707,517. Multiplex pulse code signaling. STANDARD TELEPHONES & CABLES, Ltd. Sept. 5, 1952, No. 22389/52. Class 40 (5) In a pulse communication system signal sample amplitudes are represented by a time modulated pulse or group of time modulated pulses representing a quantized value of the sample and a further time modulated pulse representing the magnitude of the difference between the actual and quantized values of the sample amplitude. In the multi-channel system described there are thirty-two channels, one of which is used for synchronizing, each having a fourmicrosecond channel period divided into two two-microsecond half-periods. During the first a "decade" pulse, which can assume any one of ten time positions, is transmitted representing the sample amplitude with an error not exceeding 5 per cent of the maximum. During the second half-period a " vernier " pulse is transmitted which can assume any position, so that the error value indicated thereby is effectively multiplied by ten. Transmitter.-At the transmitter a master generator 1, Fig. 1, produces a train of pulses with a recurrence frequency of 5 Mc/s, which are fed to a frequency divider 2, dividing by twenty. The latter produces trapezoidal pulses A, Fig. 2, of duration two microseconds defining the first halves of the four microsecond channel periods shown in graph D. A further frequency divider 4, dividing by two, produces two trains B, C, of two-microsecond sawtooth pulses, the train B being phased to occupy the second halves of the even-numbered channel periods while the train B are associated with the odd-numbered channel periods. The individual channel units 11, 14 ... 18 are gated by pulses from tappings 12, 19 ... 23 on a delay line distributer 9 fed with four-microsecond pulses at channel recurrence frequency from the frequency divider 7 which divides by sixteen the output from divider 4. The channel units are fed with modulating signals at terminals 24 ... 28 respectively and with the A pulses from divider 2 and produce duration modulated pulses terminating at the end of the first half of the channel period in the manner described in Specifications 587,939 and 635,472, also Fig. 4 (not shown). The synchronizing channel, circuit 11, produces a one-microsecond pulse 33, graph D, in the second half channel period, from the sawtooth pulse C on conductor 6. All the even-numbered channel circuits are connected to a decade modulator 29 and vernier modulator 30, the odd-numbered circuits feeding a second decade modulator 31 and vernier modulator 32. The outputs of the synchronizing channel circuit and all these modulators are mixed in circuit 13 and fed to the radio transmitter 36. The decade modulators 29, 31 are gating circuits which pick out the first, e.g. 34, 81 graph D, of the 5 Mc/s pulses from generator 1 which occurs during the durationmodulated pulse from the channel units connected thereto. They also pass on to the vernier modulators a pulse whose duration depends upon the error in the decade pulse which is translated into the time position of an output pulse occurring in the second half channel period, e.g. 35, 82, graph D. Vernier modulator 30 is also supplied with the B pulses on conductor 5 and modulator 32 with the C pulses on conductor 6, while both receive the A pulses on conductors 3. Receiver.-At the receiver, the synchronizing and channel pulses are recovered by the radio receiver 39, Fig. 3. A timing pulse generator 43 is supplied through a gate circuit 40 with the decade pulses only, which cause a series of sharply resonant circuits to ring and produce a noise-free sine wave of 5 Mc/s, which is converted into short pulses of the same recurrence frequency. These are fed to a frequency divider 44 which divides by twenty and produces two trains E, F, Fig. 2, of two-microsecond pulses phased to correspond respectively with the first and second halves of the channel periods. The E pulses are fed to a divider 47 which divides by two to provide two trains G, H, of fourmicrosecond pulses corresponding respectively with the odd and even numbered channel periods. The E pulses are supplied as gating pulses to circuit 40. The synchronizing pulses are selected by circuit 41 and synchronize a gate pulse generator 51 which receives the F pulses from circuit 44 and divides their recurrence frequency by thirty-two. The gating pulses are fed to a delay line distributer 52 controlling the channel gating circuits 53 ... 57. The latter receive reconstituted duration modulated pulses corresponding to the channel decade and vernier pulses and derived from circuit 64 ... 70. These duration modulated pulses are passed to their appropriate channel output circuits through low pass filters 73 ... 77 respectively to reproduce the modulating signals. Circuits 64, 67, gated by pulses E, F respectively, select the decade and vernier pulses. The latter are converted to duration modulated pulses 79, Fig. 2, graph J, in circuit 68, attenuated to one tenth of their amplitude 80, Fig. 2, in circuit 69 and passed to the mixer 70. The decade pulses are trimmed to remove noise, by circuit 65 under the control of 0.15 microsecond pulses at 5 Mc/s from generator 72 fed from source 43, and then converted to duration modulated pulses 78, Fig. 2, in circuit 66, which feeds them to mixer 70. Decade modulator.-The decade modulators 29, 31, Fig. 1, each comprise a circuit as shown in Fig. 5, explained with reference to the graphs of Fig. 7. The gating valve 122 has applied to its suppressor grid the 5 Mc/s pulses 134, graph K, the duration modulated pulses 133, graph L, from the channel units being applied to its control grid. The first of the 5 Mc/s pulses passed by valve 122 triggers the multivibrator 83, 84 which is self-restoring after about two microseconds to produce pulse 135, graph M, at the cathode of valve 84. The circuit 118, 119 differentiates this pulse, the resulting positive leading edge pulse at terminal 117 forming the decade pulse 137, graph N. A negative pulse, the inversion of pulse 135 is fed from the anode of valve 84 to the vernier modulator via terminal 120. Vernier modulator.-The vernier modulators 30, 32, Fig. 1, each comprise a circuit as shown in Fig. 6. The pulses 133, graph L, Fig. 7, from the channel unit and the inverted pulse 135, graph N, from the decade modulator are applied respectively to the control and suppressor grids of valve 138. At the anode thereof appears a pulse 146, graph P, commenced and terminated respectively by the leading edges of said two pulses and whose duration thus represents the error in the amplitude represented by decade pulse 137. After inversion in transformer 143, pulse 146 is applied to a cathode follower charging valve 148 which feeds it into a condenser 150 through capacitor 151 and rectifier 152. The potential 161, graph Q, acquired by the condenser is proportional to the duration of pulse 146. This potential is applied to a cathode follower valve 155 and thence to a combining valve 157 through the secondary of transformer 158. The primary of this transformer is fed with the sawtooth pulse B, also shown as 160, graph R and the resultant voltage variation 165 on the grid and cathode of valve 157 is shown by graph S. The cathode is connected to a pulse modulator 163, which produces a duration modulated pulse 166, graph T, the leading edge of which occurs at a time depending on the duration of pulse 146. The circuit of the modulator 163 is similar to that of the channel units 14 ... 18, Fig. 1, and Fig. 4 (not shown), and includes differentiating means to produce a sharp leading edge pulse 167, graph U, which forms the vernier pulse to be transmitted. The condenser 150 is discharged by applying the A pulse through terminal 168 to valve 154, the inverted pulse at the anode being differentiated by circuit 171, 172 and the negative leading edge pulse 174, graph V, being fed through rectifier 173 to neutralize the charge in condenser 150. Specification 535,860 also is referred to. Reference has been directed by the Comptroller to Specification 687,063.